Electromagnetic devices such as various kinds of transformers and reactors are widely used in power supply and distribution systems. Reduction of their cost and/or power losses can significantly improve the economic parameters of such power systems.
In electromagnetic devices, power losses in the windings are directly proportional to the square of the loading of the winding. Therefore, power losses in a winding are much lower under low load conditions than under heavy load conditions. To the contrary, power losses in the core of an electromagnetic device having a ferrous core are independent of the load, and therefore power losses do not change significantly as long as the device is connected to the power system. This can be costly, because in many applications the devices are always connected to the power system regardless of whether there is load on them or not.
Conventional methods for reducing losses in a ferrous core have involved the use of higher quality steel for the core. For example, a major advancement in core losses reduction was the introduction of cold rolled grain-oriented steel. Grain-oriented steel has a polycrystalline structure, which provides high permeability and low energy dissipation (power losses) when the magnetic field flows in the direction of the grains.
However, there are two main drawbacks in the use of the grain-oriented steel. The cost of grain-oriented steel is substantially higher than the cost of non-grain-oriented steel; and the power loss in grain-oriented steel is substantially higher when the flux is flowing perpendicular (quadrature) to the direction of the grains than when the flux is flowing in the direction of the grains. As a result, a relatively high power loss is concentrated in the corners of a ferromagnetic core where the flux direction changes and crosses the grain orientation, as illustrated schematically in FIG. 2. The higher the grade of the grain-oriented steel, the higher is the difference between the power losses with flux flowing along the grain orientation versus across the grain orientation.
In order to reduce such power losses in the corners of grain-oriented steel cores, the prior art employed different core configurations such as mitered cores and wound distributed-gap cores. The use of a mitered core allows for some reduction of corner losses, but at significantly greater expense than a conventional grain-oriented steel core. A wound core is even more expensive than a mitered core, and in general for multi-phase devices does not result in any substantial reduction of power losses in the core.
The highest level of core losses reduction is achieved through use of amorphous steel for the core. However, the cost of amorphous steel is extremely high, and as such this core design option is not widely used.